You've been given good advice. Like the other indicate: Preparation is important.

A couple of my notes:
-brumation area needs to be dark.
-Check whether the temperature you'll have in the brumation room is not to high, nor to low. Not all garters need to be brumated at very low temperature's ( check the caresheet articles ), but for a few it's quite good to go to temperature like 6°C.
-Wait at least 2 weeks before you take them in brumation ( I personally wait 3 weeks )
-drop the temperature gradually and shorten the light cycle gradually ( I personally do about 2,5-3 weeks about it ).
-Once they are brumating, don't disturb them to often. But it's good to check on on them like every week or if you're more sure every 2 weeks. Like Ameivaboy says, you might need to bring them out of brumation is they loose to much weight. Last year the temperature stayed way to high during a long period and I also took them out to early for my norms.
-I also provide a water bowl and I make a dry and a 'wet' part ( humid, not soaked ).
- brumate them for at least 6 weeks ( I brumate adults for a longer period if the temperature allows it, 2 - 2,5 months )
-bring them out of brumation by gradually increasing the temperature and light cycle.